One known boiling water nuclear reactor includes a drywell, a wetwell, a Gravity Driven Cooling System (GDCS) and a passive containment cooling system (PCCS). The drywell is designed to contain pressure resulting from a Loss-Of-Coolant Accident (LOCA), and the PCCS is configured to remove core decay heat following a LOCA and to limit the pressure within the reactor containment to a pressure below a design pressure of the containment during a LOCA.
The GDCS is substantially isolated from the drywell and is an emergency source of low pressure reactor coolant used following a loss of coolant event in at least one known boiling water reactor (BWR). A typical GDCS includes pools of coolant positioned so that when coolant from the pools must be supplied to the RPV, the coolant flows, under gravity forces, through the GDCS coolant delivery system into the RPV. Under normal reactor operating conditions, however, coolant from the GDCS does not flow into the RPV.
A typical PCCS includes several condensers positioned in a PCCS pool, or pools, of water. Each condenser includes an upper drum, a lower drum, and several heat exchanger tubes extending between the upper and lower drums. The upper drums are coupled to the drywell via a steam inlet passage, and steam generated within the containment and noncondensible gases flow from the upper drums and to the lower drums through the exchanger tubes. The steam is condensed into water and the condensed steam is drained from the lower drums and to a condensate drain tank via a condensate drain line.
The noncondensibles are purged from the lower drums utilizing vent lines. Particularly, a vent line extends from each lower drum and into the wetwell so that the noncondensibles collect in the wetwell. To condense any steam that might flow through the vent line and not through the condensate drain line, e.g., during a blowdown, one end of each vent line is submerged in the suppression pool.
The wetwell is separated from the containment drywell by a wall having an opening therein. A vacuum breaker typically seals the opening and is movable between an open position and a closed position. The vacuum breaker is a check valve which allows fluid to pass from the wetwell to the drywell to substantially prevent a large differential pressure from developing between the wetwell and the drywell. Particularly, if pressure in the wetwell becomes sufficiently great compared to pressure in the drywell, the vacuum breaker opens and allows fluid to pass from the wetwell to the drywell and reduce the differential pressure.
If the vacuum breaker becomes stuck in the open position, it is possible for the differential pressure between the wetwell and the drywell to reduce too much. Particularly, it is possible for the differential pressure to be insufficient to force noncondensibles to flow from the PCCS to the wetwell. The noncondensibles, accordingly, could build up in the PCCS and render the PCCS inoperable.
It is known that one way to prevent a vacuum breaker from sticking in the open position, is to utilize an isolation valve. However, isolation valves sometimes fail, which causes the vacuum breaker to cease operating. In addition, the isolation valve must often be monitored to ascertain whether it is working properly.
It would be desirable to provide a system facilitating the removal of noncondensibles from the PCCS even while the vacuum breaker is in the open position. It further would be desirable for such system to facilitate the maintenance of an acceptable drywell to wetwell pressure differential.